1. Field of the Invention
The present invention generally relates to a rotary airlock for use in a pneumatic conveyance system which delivers material to a pneumatic pipeline while preventing loss of air pressure therefrom. More specifically, the invention relates to a rotary airlock which includes an improved sealing mechanism which minimizes pressure and air losses about a rotor assembly in the airlock.
2. Description of Related Art
Pneumatic conveyance systems are generally utilized to convey granular, pelletized and particulate material. The conveyance system includes a pipeline through which an air stream is forced. Particulate material is delivered to and becomes entrained within the air stream to effect conveyance. In the past, rotary airlocks have been utilized to feed pneumatic conveying pipelines. Throughout the subject application, the term "airlock" is utilized to refer to a device which is able to move material continually between inlet and outlet ports while simultaneously maintaining an air pressure or vacuum differential between the inlet and outlet ports. Rotary airlocks are capable of moving various materials between the inlet and outlet ports, regardless of whether the airlock experiences an increased pressure or vacuum at the inlet port, or outlet port, or both.
The airlock includes a rotor assembly (also referred to as a paddle wheel, impeller or flipper) formed from a combination of blades and a shaft welded or assembled together. The blades (also referred to as vanes) represent metal plates formed in a "pie-shaped" cross-section to form pockets therebetween. The pockets between the vanes carry the material as the rotary airlock is operated. As the vanes and pockets rotate from the inlet port to the outlet port, the vanes and pockets are referred to as being on the "load side" of rotation and as the vanes and pockets pass through the discharge region proximate the outlet port, they enter the "return side" of rotation. The vanes and pockets are thereafter moved through the return side of rotation to a position in communicating relationship with the inlet section. The rotor assembly is rotatably received within a housing having the same contour as the rotor vanes. The vanes pass along the interior surface of the housing during rotation and maintain a sealed relation therewith. This sealed relation, to a significant extent, dictates the rotary airlocks ability to maintain a sealed relation between the inlet and outlet ports.
However, past rotary airlocks have offered poor efficiency, excessive maintenance, excessive operator manpower, production degradation, equipment wear and replacement, and lost production due to processing shut downs.
In the past, rotor assemblies have been proposed with differing numbers of vanes to enhance the sealing ability of the airlock proximate the outer edge of the vanes. In particular, a rotor assembly having ten vanes maintains at least four vanes in close contact with the housing at all times, while a rotor assembly having six vanes is only able to maintain two vanes in close contact with the housing at all times. Hence, the ten vane rotor maintains a better sealed relation with the housing. However, as the number of vanes increases, the capacity of each pocket decreases. Additionally, the opening to each pocket narrows, thereby restricting the flow of particulate material into the pocket when such material is of a "slow flowing" nature. Also, rotors having six vanes are capable of handling larger products such as peaches, potatoes, carrots and other food products better than rotors having ten vanes.
In the past, rotor assemblies have been constructed of two primary types, namely open ended and close ended rotors. An open ended rotor assembly represents one in which the pockets are defined at opposite ends by the end plates of the housing. In an open ended rotor, opposite ends of each vane rotate in close noncontacting proximity to the end plates of the housing. Hence, rotors of the open ended type include vanes having three wear surfaces (also referred to as three tip areas) and three surfaces along which pressurized air may escape from the airlock. In most open-ended rotor designs, the seal between opposite ends of the vanes and the end plates of the housing is directly exposed to the material and is the sole mechanism for preventing the material and air from passing about the ends of the vanes. Thus, the ability of the rotary airlock to maintain the material and air pressure within a pocket without leakage depends upon the performance of the seal. Such seals have proven disadvantageous as they permit excessive air loss about the rotor as the vanes wear.
To reduce air loss and wear, an enclosed end rotor assembly has been proposed which includes disk-shaped plates attached to opposite ends of each vane to enclose the ends of each pocket. The end plates are generally referred to as "shrouds" and may be secured to the vanes through welding, molding and the like. The shrouds enclosing opposite ends of each vane reduce the wear surfaces upon the rotor assembly to a single edge along the exterior surface of the vane and along the outer perimeter of the shroud which slidably communicates with the housing. Hence, the shrouds significantly reduce the wear area upon each vane.
The rotor assembly is formed to operate in a close tolerance with the inside diameter of the housing. The vane tips and the shroud tips are formed with an outer diameter located immediately adjacent the inner diameter of the housing bore. The clearance between the vane tips, shroud tips and the housing bore is preferably a few thousands of an inch. Past enclosed end rotor assemblies have been proposed which are formed with tapered vanes combining to form a rotor with a larger diameter at one end than the diameter at the opposite end. Similarly the interior tubular contour of the housing is formed in a cone shape with one end of the housing having a large interior diameter than an opposite end of the housing interior. This tapered configuration enables the tolerance between the rotor and interior of the housing to be adjusted by laterally shifting the rotor within the housing. This lateral adjustment compensates for wear, thermal expansion and other factors that affect the sealing efficiency of the rotor. The end shrouds upon a closed end rotor enable the rotor to be laterally shifted, while maintaining enclosed ends for the pockets.
The enclosed end rotor assembly, when assembled in the rotary airlock housing, maintains a gap or clearance between an outer surface of each shroud and the inner surface of the adjacent housing end plate. The gap enables the rotor assembly to be centered between the end plates of the housing or laterally shifted toward either end plate as needed. Thus, the gap distance between shrouds and end plates may be the same on both ends of the device or it may be greater on one end than the other.
In the past, several types of rotor tips have been proposed to minimize air loss between the vanes and housing, namely plain tips, fixed-relieved tips, adjustable tips and the like. Plain tips are constructed with a slight arcuate or convex surface formed along an arc corresponding to the interior curvature of the housing. Fixed relief tips include a narrower width proximate the tip area, as compared to a plain tip, by forming a beveled edge receding away from the tip in the direction of rotation. Adjustable tip designs include an add on vane tip that is bolted to the vane and designed to be adjustable closer to or away from the airlock housing. While these rotor tip designs attempt to minimize air loss between the vanes and housing, air may still escape through the clearance between the outer diameter of the shroud and the inner diameter of the housing. In particular, air may travel from the pockets proximate the outlet port of the valve through the clearance between the shroud and housing and into the gap between the shroud and end plate. The air passes upward along the end plate and back through the clearance between the shroud and housing into the pockets proximate the inlet port of the valve. Hence, air loss occurs about the ends of the rotor assembly by migrating between the inlet and outlet ports via the gap between the shroud and end plates.
In the past, a shroud seal assembly has been proposed for preventing air loss about the shrouds. The conventional shroud seal may be formed of a compressible packing, a leather lip, an elastomer, or a synthetic rubber. The shroud seal engages the outer surface of the shroud to prevent air migration therealong. The shroud seal is biased against the shroud with a plurality of mechanical springs distributed evenly about the rotational axis. The springs include one end mounted to the end plates and an opposite end biased against the shroud seal. The springs force the shroud seal against the shroud. The shroud seal operates in a close tolerance with the housing.
However, the foregoing mechanical spring type shroud seal has experienced several limitations. The biasing force induced by the mechanical springs varies dependent upon degree to which the spring is contracted. Hence, when the rotor assembly is laterally shifted to adjust the vane/housing clearance, the gaps between the shroud and end plates vary, with a first shroud gap decreasing while a second opposite shroud gap increases. Accordingly, the set of springs for the first shroud gap is compressed, while the set of springs for the second shroud gap is expanded. The compressed set of springs induces increased sealing force upon one shroud seal, while the expanded set of springs induces less sealing force upon a second shroud seal. Such variations in the shroud sealing force are undesirable, as it compromises the seal or apply excess friction upon the shroud. Hence, the mechanical springs must be manually adjusted every time the rotor assembly is shifted laterally.
Further, the conventional shroud seal includes an outer diameter which does not seal optimally against the inner diameter of the housing. Hence, air loss occurs between the shroud seal and housing. This air migrates past the springs and ultimately is lost about the backside of the shroud seal and through the inlet port. The shroud seals are unable to effectively seal against the housing since the springs only bias the shroud seals against the shroud and not radially against the housing. The shroud seals are formed with a constant outer diameter which is not adjustable radially and hence is unable to maintain a seal with the housing as the rotor and shroud seals are shifted laterally.
In the past, rotor shaft seals have been proposed, such as lip seals, and packing gland seals to prevent air pressure loss about the rotor shaft. However, conventional shroud seals do not effectively prevent abrasive materials that can migrate to the seal area destroy the rotor shaft seal quickly.
A need remains within the industry for an improved adjustable pneumatic seal for a rotary airlock valve which is self-aligning and requires low maintenance. It is an object of the present invention to meet this need.